Process for the photochemical chlorination of benzene



3,006,833 Patented Oct. 31, 1961 3,006,833 PROCESS FOR THE PHOTOCHEMRCAL CHLORINATION (3F BENZENE Aylmer H. Maude and Madhav R. Bhagwat, Niagara Falls, Carson E. Lisman, Lewiston, and David S. Rosenberg, Niagara Fails, N.Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Sept. 1, 1953, Ser. No. 377,963 1 Claim. (Cl. 204-163) This invention relates to an improved process for the photochemical chlorination of benzene in sym-tetrachloroethane also known as acetylene tetrachloride at low temperatures and to the recovery therefrom of benzene hexachloride product enhanced in its gamma isomer content.

The use of solvents in the chlorination of benzene to obtain higher yields of the gamma isomer of benzene hexachloride has been known for several years. For instance, British patent application 36,524, filed on December 11, 1946, and British Patent 656,457 show the use of solvents in addition chlorinations of aromatics when the halogenation initiator is light; also British Patent 653,364 shows the use of solvents in the chlorination of benzene when the halogenation initiator is a peroxide.

Although it has been known for some time that chlorination of benzene at low temperatures will reduce the relative amounts of substitution products, it has been only recently that anyone realized lowering the temperature would also increase the yield of gamma isomer. Gonze, U.S. Patent 2,513,092, issued February 27, 1950, shows a method for obtaining a benzene hexachloride free from substitution products and having improved insecticidal qualities by the photochemical chlorination of benzene at temperatures below six degrees centigrade (e.g. minus 20 C.) in an organic solvent of the aliphatic series which is a liquid at six degrees centigrade and is substantially indifferent to chlorine under operating conditions. Carbon tetrachloride is the only solvent mentioned in the patent specification, but tetrachloroethane and chloroform are additionally suggested in the file wrapper of this patent as open to the public. Belgian Patent 471,772 shows the photochemical chlorination of benzene at low temperatures in the presence of an aliphatic polyha1ogenated compound such as chloroform. This patent asserts a gamma isomer yield of forty percent but the disclosure is very vague in the chlorinating conditions used and in the method of recovery and analysis of the product. Saur (Chemical Abstracts, vol. 44, p. 5827 (1950)), from a consideration of a theory of the mechanism of the reaction, postulates that lower temperatures will increase the yield of gamma, but shows no experimental vertification of his postulation.

Currently Roels et al., British Patent 678,577, show their work in the photochemical chlorination of benzene in various partially chlorinated aliphatic hydrocarbon solvents at temperatures below zero degrees centigrade to obtain yields of the gamma isomer such as twenty percent.

Since the discovery of the insecticidal activity of the gamma isomer of benzene hexachloride, prior investigators have been concerned with obtaining a crude benzene hexachloride product having the highest possible concentration of gamma isomer, such product to be used directly in insecticidal formulations or as a raw material for making more concentrated gamma isomer compositions including the substantially pure gamma isomer known as Lindane. Because of the desirable characteristics of insecticidal formulations made directly from Lindane as compared to those having the same gamma content but made from crude technical benzene hexachloride compositions, the demand for Lindane has increased and many investigators are pursuing improved processes for making it. It has been found that variations in the relative distribution of the benzene hexachloride isomers in crude benzene hexachloride compositions seriously infiuence the results which may be obtained in converting such compositions to more concentrated gamma benzene hexachloride compositions or to Lindane. The main relationships in the distribution of isomers which are important in this respect and which are desirable to control are the ratio of gamma to alpha isomer and the ratio of gamma to delta isomer in the product produced. It has been found that in order to obtain optimum results in processes for making Lindane in which these factors are important that these ratios must be maintained as high as possible in the starting benzene hexachloride composition.

It is therefore an object of this invention to produce benzene hexachloride compositions having a high percentage of gamma isomer and which also contain the desirable high ratios of gamma to alpha isomer and gamma to delta isomer. A second object is to provide a method of addition chlorination of benzene in the presence of sym-tetrachloroethane in which the relative distribution of benzene hexachloride isomers may be controlled. A third object is to provide a method for enhancing the gamma isomer content of the benzene hexachloride prodnot, to form a fortified benzene hexachloride material which has an isomer distribution suitable for further processing to the pure gamma isomer therefrom. A fourth object is to provide a continuous process suitable for commercial operation which requires a minimum of labor and handling costs.

These and related objects are accomplished by the present invention which comprises effecting the photochemical reaction between chlorine and benzene in a reaction medium comprising a substantially single liquid phase system of sym-tetrachloroethane having dissolved therein between about two and fifteen percent of unreacted benzene, maintaining said reaction medium substantially free of reaction inhibitors and at a temperature between about plus 10 degrees centigrade and about minus 25 degrees centigrade, maintaining the light intensity and chlorine feed rate into said medium in relationship to an isomer of benzene hexachloride in the reaction medium to pro duce a benzene hexachloride composition having a gamma isomer content on the dry basis of more than 20 percent of the total weight of benzene hexachloride isomers and having a ratio of gamma to delta isomer of above 2.0 to 1.0; and separating the benzene hexachloride composition so produced by evaporating the sym-tetrachloroethane and unreacted benzene. This invention also cornprises continuing the processing by evaporating from the reaction medium sym-tetrachloroethane and unreacted benzene to cause the precipitation of alpha and beta isomers of benzene hexachloride and to concentrate the gamma isomer retained in solution; continuing said evaporation until the gamma isomer is between 70 and percent of saturation in the solution at the temperature employed for separating the precipitated alpha and beta benzene hexachloride isomers from the solution containing the dissolved gamma isomer; separating the solution containing the dissolved gamma isomer from the precipitated alph and beta isomers; and recovering therefrom by evaporating substantially all the remaining sym-tetrachloroethane, a benzene hexachloride composition containing more than 50 percent by weight gamma isomer.

We have found that the reaction between chlorine and benzene is very sensitive to impurities and care should be taken to be sure the starting materials are free from harmful inhibitors or materials which might prevent the reaction between benzene and chlorine, or alter the relative distribution of the benzene hexachloride isomers which are produced. The presence of free oxygen or certain oxygenated organic compounds in the system will inhibit the chlorination. Experiments in which hydrogen chloride gas was used as a diluent for the chlorine showed that products having lower percentages of gamma result than when the chlorine gas was either undiluted or diluted with carbon dioxide or nitrogen. Hydrogen chloride may also be present in the chlorination mixture because of slight substitution chlorination of the benzene, benzene hexachloride isomers and acetylene tetrachloride. These traces of hydrogen chloride can be purged from the system by using an inert gas purge, such as oxygen-free carbon dioxide or nitrogen. By suspending finely divided anhydrous sodium carbonate in the reacting mixture oxygenated organic compounds and hydrogenchloride will be removed, but it has been found that where the starting materials have been properly pro-treated, these continuous treatment methods are not necessary. The acetylene tetrachloride and benzene feed materials should be pre-treated with finely divided precipitated magnesium oxide, magnesol (a highly absorbent magnesium silicate), and calcium chloride or sodium sulfate and filtered, to remove traces of hydrogen chloride, traces of impurities (such as carbon, oxygenated organic compounds and iron), and traces of water, respectively. If these pretreated materials are used soon after treatment, the use of a method for oxygen removal may not be necessary during actual chlorination, but if the starting'materials are allowed to stand and pick up dissolved oxygen, a method for oxygen removal during chlorination may be necessary. The methods of removing oxygen which may be advantageously employed are continuous oxygen free the concentration of the activated chlorine atoms will not 7 be as uniform throughout the liquid as when using light of longer wave lengths. Because of these effects it is preferred to use light having wave lengths well above 4000 Angstroms in order to obtain a more uniform chlorine activation throughout the mixture and to reduce the amount of light energy needed to effect the chlorination. However it has been found that the isomer distribution in the benzene hexa-chloride produced seems to be independent of the frequency of the activation energy when the proper uniform reaction conditions are maintained in accordance with our invention.

It also has been found that the chlorination reaction is somewhat, dependent on the particular apparatus or equipment. in use. Wall effects .of the irradiation zone and variation in depth of solution for light penetration greatly influence the chain length of. the. free radicals formed during the activation of the chlorine and its reacinert gas purges of carbon dioxide and nitrogen, or pretem, should be purged of oxygen and other impurities in order to obtain a substantially pure chlorine feed supply.

Where inert nitrogen or carbon dioxide atmospheres or purges are used, these gases should likewise be oxygenfree.

The benzene concentration of the benzene-acetylene tetrachloride feed mixture should be small but may be varied from between 2 and 15 percent benzene in the mixture, with concentration between 5 and 10 percent being preferred. Where smaller concentrations of benzene are used, the process becomes uneconomical to operate, and where benzene concentrations much over 15 percent are used there are increases in the beta and delta isomers at the expense of the gamma isomer.

The chlorination'should be effected under substantially single liquid phase, conditions. The chlorine gas should be dissolved into the benzene-acetylene tetrachloride mixture before introduction in the irradiation zone and the reaction should be conducted such that very little or no solids precipitate out to form a substantial amount of a suspended second phase for these suspended solids interfere with the depth of penetration of the radiation into the solution. Small amounts of solids appear to have little effect on the relative isomer distribution and as such may be tolerated.

The wave length of the activation energy source should be Within the region of continuous absorption for the chlorine molecule, that is, between 2500 and 4800 Angstroms. However, since glass absorbs light having wave lengths below 3000 Angstroms, these wave lengths have little elfect when the chlorine is separated from the source of irradiation by glass. I Further from a theoretical standpoint, it has been found that when using light of wave lengths in the upper range of available wave lengths, namely, from 4000-4800 Angstroms, less light energy is required to activate the chlorine molecules than when light of lower wave lengths is used for the quantum efliciency is greater at the higher frequencies. Also, light of tion with benzene and appear to somewhat alter the results whensubstantially identical experiments are carried out in different irradiation zones.

The temperature'has a great elfect 'on the-maximum amount of gamma isomer which can be obtained but appears to have very little eifect on the amount of delta isomer formed. Table 1 shows the effect of lowering the temperature on obtaining the maximum yield of the gamma isomer. However, as the temperatureis lowered the quantum efiiciencies of the irradiation energy drops; thus requiring either more irradiation energy or less chlorine feed to bring the chlorination conditions into the desirable range of operating conditions. Further, at these lower temperatures, if the irradiation energy input is not increased, the traces of inhibitors still remaining in the system will have a more appreciable effect in retarding the TABLE 1 Efiect of temperature on benzene hexachloride isomer distribution under conditions of maximum:

The amount of benzene converted to benzene hexachloride may be varied withinwide limits without affectmg the relative'isomer distribution of the product. Conversions of as low as 10 percent and as high as percent have shown insignificant changes in the relative isomer distributions. However, the amount of solids does appear to have significance at high concentrations, especially those concentrations where substantial amounts of the benzene hexachloride isomers are suspended in so lution forming a substantial'quantity of'a second phase, it

becomes more difficult toymaintain uniform chlorination conditions and erratic results are obtained. At lower concentrations than the desired range the reaction be comes uneconomical to operate. It has been found that the maximum desirable range of solids is between and 18 percent and a preferable range is between 12 and 15 percent solids.

We have further found that when operating within the preferred chlorination conditions of this invention, the relative distribution of the benzene hexachloride isomers can be controlled to within certain limits by adjustment of either the light intensity or the chlorine feed rate or both by maintaining the light intensity and chlorine feed rate in relationship to an isomer of benzene hexachloride in the reaction medium. This is especially desirable for obtaining an optimum gamma to delta ratio in the chlorinated product. The change in relative isomer distribution with change in light intensity is demonstrated in Table 2, which shows the results of several experiments using the same chlorination equipment and substantially identical chlorination conditions except for the conditions shown in the table. Each example was made as follows:

A 500 milliliter, 3-necked glass, round-botom flask equipped with agitator, gas inlet tube and vent gas exit, was immersed in a methanol-Dry Ice bath contained in a glass beaker. The light source was supplied to the flask through the bottom of the beaker and flask. grams of benzene pre-treated with calcium chloride, 380 grams of freshly substantially pure sym-tetrachloroethane, pretreated with sodium sulfate and magnesol, were charged into the flask, cooled and maintained at a constant temperature throughout the reaction with agitation. A mercury vapor lamp was turned on and its intensity measured. Carbon dioxide was bubbled into the solution at the rate of 400 cubic centimeters per minute for 10 minutes to prepurge the solution of gaseous inhibitors. The carbon dioxide feed was continued at the same rate and chlorine, pre-gased to purge the lines of oxygen, was fed with the carbon dioxide into the mixture at the rate of 290 cubic centimeters per minute (50 grams per hour) for 50 minutes, at which time the reaction was stopped and the reaction mixture placed on a steam bath and an air stream was used to sweep out the resulting vapors of solvent and unreacted benzene. The benzene hexachlo ride solids were then alalyzed by infra-red absorption, the percent beta isomer being reported by difference.

The information contained in Table 2 illustrates several important novel findings of this invention which we have found essential to follow in order to obtain successful results in accordance with our teaching. At constant temperature, operating conditions and apparatus, the alpha isomer yield increases with increase in relative light intensity, the gamma isomer yield passes through a maximum value with increase in light intensity, the delta isomer yield decreasesin light intensity, and the ratio of gamma to delta isomers increases with increase in light intensity. Further, it has been discovered as partially shown in Experiment 19 when compared with Experiment 7 that at constant temperature, operating conditions and apparatus, the alpha isomer yield decreases with increase in chlorine feed rate, the gamma isomer yield passes through a maximum value with increase in chlorine feed rate, and the delta isomer yield increases m'th increase in chlorine feed rate. Thus, in accordance with our invention the relative isomer distribution may also be varied by changing the chlorine feed rate while maintaining the light intensity and the operating conditions and apparatussubstantially identical. This method is preferred for it is more readily adaptable to practical commercial plant operation. In this method, a chlorination run would be made at an arbitrarily set chlorine feed rate and after conditions had become substantially steady; a sample of the reaction product would be withdrawn and analyzed primarily to determine the relative isomer distribution of the alpha, gamma, and delta isomers. If the gamma to delta ratio was near 2.0 to 1.0, the chlorine feed rate would then be decreased, but if the gamma to delta ratio approached 4.0 to 1.0 or higher, the feed rate would then be increased. Thus, if the chlorine feed rate is too high giving a low gamma to delta ratio, the feed rate should be decreased, all other operating conditions and equipment remaining substantially the same; similarly, if the chlorine feed rate is too low giving a high gamma to delta ratio, the chlorine feed rate should be increased. Such a method of control is readily adaptable to present day instrumentation which permits periodic adjustment of operating conditions based on alaysis for an isomer of benzene hexachloride in the reaction medium and correlating the results with the desired operating range which has previously been determined for the particular apparatus in use. Thus, in Table 2, Example 10, when the light intensity was arbitrarily set at the rel ative value of 157 foot candles and the chlorine feed rate arbitrarily set at 50 grams per hour, the benzene hexachloride isomers in the reaction mixture were analyzed by infrared spectrometry and found to be in the following distribution: alpha, 57.8 percent; gamma, 21.2 percent; delta, 10.1 percent; epsilon, 1.9 percent; and beta, 9.0 percent (by difference). This gave a rather low gamma isomer content and a high delta isomer content, when compared with what could be obtained at minus 10 degrees centigrade, and a rather low gamm-a-to-delta ratio. However, in accordance with our invention by adjusting the operating conditions of Example 10 to those of Example 7 of Table 2 wherein the chlorine feed rate is maintained the same as that of Example 10, ie. 50 grams per hour, but the relative light intensity is increased to 1830 foot candles, the benzene hexachloride isomers in the reaction mixture were analyzed by infrared spectometry and found to be in the following more desirable distribution: alpha, 62.4 percent; gamma, 25.1 percent; delta, 6.3 percent; epsilon, 2.8 percent, and beta, 3.4 percent (by difference), the gamma-to-delta ratio being 3.98. And for instance, in Example 19 of Table 2, when the light intensity was arbitrarily set at the relative value of 1830 foot candles (the same as used in Example 7) but the chlorine feed rate arbitrarily set at 12.5 grams per hour, the benzene hexachloride isomers in the reaction mixture were analysed by infrared spectrometry and found to be in the following distribution: alpha, 69.7 percent; gamma, 21.4 percent; delta, 5.6 percent; epsilon, 2.1 percent; and beta, 2.0 percent (by difference), with the gamma-to-delta ratio being 3.82. This gave a rather low gamma and a high alpha isomer content when compared with what could be obtained at minus 10 degrees centigrade. However, in accordance with our invention, by adjusting the operating conditions of Example 19 to those used in 'Example 7 wherein the relative light intensity is maintained the same as that of Example 7, i.e. 1830 foot candles, but the chlorine feed rate is increased to 50 grams per hour, the distribution of the benzene hexachloride isomers is again more desirable.

When both the light intensity and the chlorine feed rate are varied, all other conditions and equipment being held substantially constant, the gamma to alpha ratio may be controlled more critically and more instantaneously than when only the chlorine feed rate or the light intensity alone is varied, and where continuous automatic sampling and analyzing of the product is practical, the coordinated variation of both the light intensity and the chlorine feed rate may be advantageous.

Further, it has been found that the most practical and preferable chlorination conditions for the largest recovery of available gamma isomer are those conditions which give a benzene hexachloride isomer distribution high in gamma isomer and low in delta isomer and thus a high gamma to delta ratio. We have found that for practical operating conditions, this ratio of gamma to delta should be above 2.0 to 1.0. Thus, the percent of gamma isomer should be high but need not necessarily be at the maximum value to give the most economical results, but in fact we have found it desirable to operate under condi- TABLE 2 Relative efiect of light intensity at various constant temperatures on the isomer distribution in benzene hexachloride Relative Constant Benzene I-Iexachloride Product Analysis Example Light Temper- Ratio,

N o. Intensity ature of Gamma/ in Foot Reaction, Percent Percent Percent Percent Percent Delta Candles 0. Alpha Beta Gamma Delta Epsilon Examples 6-18.

tions such that a much lower percentage of the delta isomer is obtained at a relatively small reduction in percent gamma isomer. Further, we have found that this optimum range of relative isomer distribution can be correlated with alpha isomer yield or with delta isomer yield. The following correlation has been found for operation between the temperature limits of plus 10 degrees centigrade and minus 25 degrees centigrade. The alpha isomer content should be kept between the limits of about 58 and about 70 percent, with an optimum range between about 60 and about 66 percent. The delta isomer content should be kept between about 3.5 and about 11.5 percent, with an optimum range between about 6.0 and about 8.5 percent. Further, we have found that this optimum range of the relative distribution of benzene hexachloride isomers can be obtained when the chlorination is effected under operating conditions such that the dissolved chlorine concentration is maintained between the limits of 0.1 percent and 0.7 percent of the reaction mixture. When operating within the preferred conditions of this invention and when the light intensity and chlorine feed rate are adjusted such that the dissolved chlorine in the reaction medium is outside these limits, the relative distribution of isomers becomes erratic and subject to inhibition and undesirable chlorination effects in addition to a reduction in yield of the gamma isomer are obtained. Thus, in accordance with our disclosure, the light intensity and chlorine feed rate into the reaction medium can be easily maintained in relation to each other to produce benzene hexachloride compositions having optimum isomer distribution.

We have found the reaction product may be treated in several ways. If a benzene hexachloride product containing only to 28 percent gamma isomer is desired, it can be obtained directly by complete evaporation of the solvent and unreacted benzene from the reaction product and recovering the solid technical benzene hexachloride therefrom. The evaporation should be conducted at slightly reduced pressures to permit lower temperatures and thereby prevent possible decomposition which may occur at higher temperatures. This technical product may be used directly for insecticidal purposes; however, in many instances a product more concentrated in the gamma isomer is required and, in those cases where a substantially pure gamma isomer is required, much further processing is required to free the gamma from the other less valuable isomers. We have found that, instead of completely removing all of the acetylene tetrachloride to obtain a technical benzene hexachloride product, a more valuable product fortified in gamma isomer can be obtained if the acetylene tetrachloride is removed in two steps spaced by a crystallization step; for further, we have found that the solubilities in acetylene tetrachloride of the alpha and beta-isomers are much lower than the gamma isomer and thus, substantial amounts of these alpha and beta isomers can be separated from the gamma isomer by partially evaporating off the solvent acetylene tetrachloride to the extent that the concentration of the gamma isomer is between 70 and percent of saturation in the remaining acetylene tetrachloride at the temperature of separation of the precipitated alpha and beta TABLE 3 Solubilities of pure benzene hexachloride isomers in symtetrachloroethane Solubility in Grams per 100 Grams sym-Tetrachloroethane Isomer We have found that the separation of the precipitated Temperature, C.: Gammazalpha ratio Temperatures as low as minus 20 degrees centigrade or as high as plus. 50 degrees centigrade may be used to effect the separation. However, it has been found that when the temperature chosen is between zero and plus 15 degrees centigrade, the ratio of game to alpha is somewhat higher and theoretically more alpha can be separated from the gamma and other soluble isomers, thus,'giving a product slightly more fortified in gamma than at temperatures outside this range. We prefer to effect the separation at a temperature within this range of zero to plus degrees centigrade. Further, We have found that at the temperatures of separation, there is a tendency of the slurry to supersaturate with respect to the alpha and beta isomers; and thus, more time is required to be certain substantially all the supersaturated alpha and beta isomers have been precipitated. Usually two to five hours are sufiicient to effect the crystallization. We have also found that by sub-cooling the slurry about 10 degrees centrigrade below the temperature of separation, holding for a period followed by warming up to the temperature chosen to effect the separation, and holding for a brief period, the supersaturation effects are reduced and a better crystal size may be formed for filtration. When using this method the gamma isomer concentration should be between 70 percent and somewhat less than 100' percent of saturation to insure against any gamma isomer coprecipitation with the alpha and beta isomers. The slurry should be agitated, for the precipitated solids have a tendency to cake if allowed to settle out.

In effecting the separation, the alpha and beta isomers may be removed by any of the known unit processes used for separating suspended solids from liquids. Filtration methods, either on a filter wheels or by centrifugal methods, are preferred for they are more readily adaptable to a commercial process. The filter cake should be washed with fresh acetylene tetrachloride to displace the gammacontaining mother liquor which has been retained in the cake. One or more washes is necessary to prevent excessive losses of the valuable gamma isomer. These wash filtrates may be blended in with the mother liquor or used directly as a solvent for the chlorination of benzene or re-cycled and combined with the chlorinator effluent product to be distilled in the first step evaporation. The filter cake, containing substantially no economically recoverable gamma isomer, may be discarded or used as a starting material for the manufacture of other valuable products such as trichlorobenzene.

The two evaporation steps should both be carried out under reduced pressure so that lower temperatures may be used; and thus prevent the likelihood of decomposition at higher temperatures. In the first step the pressure should be between 2 and 60 millimeters of mercury absolute with a preferred range being between and 50 millimeters of mercury. These pressures allow for temperatures which fall between 45 degrees centigrade and 80 degrees centigrade and give a preferred range of temperatures between 50 and 75 degrees centigrade. The second evaporation step should also be conducted under pressures similar to those in the first evaporation step and should have a preferred range between 20 and 50 millimeters mercury absolute and the temperature should not go above 150 degrees centigrade is driving off the last remaining acetylene tetrachloride. The acetylene tetrachloride evaporated from these two steps is condensed and pre-treatment to remove inhibitors and other harmful impurities may be re-cycled as solvent for chlorination of more benzene or may be used in the separation step to displace the mother liquor from the precipitated alpha and beta isomers. The molten benzene hexachloride product which remains after the acetylene tetrachloride has been removed in the second evaporation step, has a melting point above 78 degrees centigrade, contains between 50 and 63 percent by weight gamma isomer and has a gamma to .delta ratio of above 2.0 to 1.0. This fortified benzene hexachloride product having enhanced insecticidal properties is especially suited for further processing to recover the substantially pure gamma isomer by extraction in a lower aliphatic alcohol, or can be used directly for insecticidal purposes.

The improved process disclosed herein can be conducted on either a batchwise or continuous basis without departing from the scope of this invention. Further, this invention embraces those processes which are in part batch and in part continuous. The chlorination step can be conducted batchwise at first followed by continuously feeding in and withdrawing of product for further processing, or the chlorination can be effected essentially batchwise folowed by direct partial or complete evaporation of the solvent acetylene tetrachloride and unreacted benzene from the reaction product.

Preferred and alternate embodiments of our invention are further demonstrated in the following examples, but we do wish to be limited thereto except as defined in the appended claims.

EXAMPLE 20 The apparatus comprised essentially a cyclic system of a pump, a heat exchanger for temperature control and a reaction zone irradiated by mercury arc lamps. The mixture of benzene and solvent pre-treated to remove inhibitors was introduced through a fiowmeter into the system ahead of the pump; and the oxygen-free chlorine gas, with or without any oxygen-free diluent inert gas, was introduced into the efiluent stream of the heat exchanger and throughly mixed with the solvent-benzene mixture before introduction into the irradiated reaction zone. The effluent reaction medium from the reaction zone was divided into two streams; one stream being bled off as product and the other being re-cycled through the system via the pump. The mercury arc lamps were adjusted to give a total illumination to the irradiation zone of about 3000 foot candles. Samples were taken from the reac tion zone effluent and analyzed for percent dissolved chlorine, percent solids, and alpha, beta, delta, and gamma isomer distribution in the solids. Before starting each run, the system was completely purged three times with an oxygen-free inert gas such as nitrogen or carbon dioxide, and during each run an inert gas blanket was maintained over the entire system to further prevent inhibition of the reaction.

Thirteen hundred grains of a mixture of 5 percent benzene dissolved in sym-acetylene tetrachloride were introduced into the system ahead of the pump, circulated through the system via the pump and brought to a constant temperature of minus 1 0 degrees centigrade by means of the heat exchanger. The reaction zone was irradiated by the mercury arc lights. Chlorine is then dissolved in the benzene-solvent mixture, then introduced into the irradiated zone very slowly at the rate of about 15 grams per hour and built up to the rate of grams per hour in about five to six minutes. The light intensity was regulated such that the effluent leaving the irradiated reaction zone contains a dissolved free chlorine concentration of between about 0.1 percent and 0.7 percent by weight. Chlorination was continued for about 45 minutes thereafter under the above conditions until the solids had built up to about 13 percent by weight. 0

The process was made continuous by slowly feeding more solvent-benzene mixture into the reaction medium at a point immediately ahead of the pump at a flow rate of about 43 grams per minute or 32 cubic centimeters per minute, and effluent reaction medium from the irradiation zone was bled off at an equivalent rate. Chlorine was continuously added at the same rate of about 160 grams per hour and the light intensity was kept such that the efiluent from the irradiation zone contained between about 0.1 percent and about 0.7 percent free chlorine. The temperature was also maintained constant throughout the reaction at minus 10 degrees centigrade. The process was run on the above continuous basis for approximately one and one-quarter hours, at which time the process was stopped. A second run was then made in substantially the same manner as above, and the products from the two runs were blended to give a composite of 2553 grams of benzene hexachloride isomers in acetylene tetrachloride (plus any unreacted benzene). A 42 gram sample of 11 this composite was removed for analysis. The total solids content were found to be 13.03 percent by weight, and the isomer distribution of these solids by infrared spectrometry was as follows: alpha, 59 percent; gamma, 24 percent; delta, 7.9 percent; epsilon, 1.9 percent; and beta, 7.2 percent (by difference).

The remaining 2511 grams of the composite of these two runs were vacuum distilled with agitation at from 20 to 30 millimeters of mercury absolute and between 42 and 60 "degrees centigrade to remove 1716 grams of acetylene tetrachloride (plus any unreacted benzene) to leave a 795 gram slurry containing approximately 327 grams of benzene hexachloride isomers and 468 grams of acetylene tetrachloride.

The slurry was agitated and cooled to zero degrees centigrade over the course of several hours and held for approximately two additional hours in order to obtain a better crystal size for filtration. The slurry was then warmed up to degrees centigrade and filtered at between 9 and 11 degrees centigrade and washed with four 113 gram portions of fresh acetylene tetrachloride. The first filter cake wash of 113 grams of fresh acetylene tetrachloride was added to the filtrate mother liquor, and the remaining wash filtrates were re-cycled and used in further chlorinations of benzene.

The filtrate contained 30.7 .grams of benzene hexachloride isomers per 100 grams of acetylene tetrachloride and analyzed as follows: alpha, 20.4 percent; gamma, 55.1 percent; delta, 19.1'percent; epsilon, 3.7 percent; and beta, 1.7 percent (by difierence). The spent cake contained 247.6 grams of benzene hexachloride isomers per 100 grams of acetylene tetrachloride and analyzed as follows: alpha plus beta, 97.98 percent; gamma, 1.25 percent; delta, 0.40 percent; and epsilon, 0.37 percent.

The filtrate mother liquor combined with the first wash were distilled at'50 millimeters mercury absolute starting at room temperature and finishing at a pot temperature of 135 degrees centigrade with agitation. After substantially all the acetylene tetrachloride had been removed, the vapor temperature dropped and the pot temperature was maintained at 135 degrees centigrade for an additional half-hour, at which time the molten product fortified in gamma benzene hexachloride was poured into dishes to solidify. This product analyzed as follows: alpha, 20.8 percent; gamma, 54.6 percent; delta, 18.4 percent; epsilon, 3.6 percent; and beta, 2.6 percent (by difference).

EXAMPLE 21 Into a 3-liter flask equipped with stirring means, thermometer, solvent benzene feed line, chlorine feed tube and sampling means, were introduced 2019 grams of acetylene tetrachloride and 120 grams of benzene which had been pro-treated to remove inhibitors. The flask was partially immersed in a temperature control beaker filled with Dry Ice and methanol, the agitator turned on and the mixture of benzene and acetylene tetrachloride was brought to a temperature of minus '10 degrees centigrade. Two H-5 mercury arc lights were placed three inches from the opposite'sides of the flask and turned on. The apparatus, lines, and liquid were purged of inhibitors with oxygen-free carbon dioxide. Chlorine gas diluted with carbon dioxide were fed into the mixture at the rates of 300 grams per hour'and 2.1 liters per minute respectively. Chlorination was discontinued after 60 minutes when 304 grams of chlorine had reacted with 112 grams of benzene to give 416 grams of benzene hexachloride isomers calculated from the gain in weight of the flask, enough makeup benzene was added to give 127 grams of unreacted benzene and chlorination was then continued as before. After minutes when 270 additional grams of chlorine had 12 1 reacted with 99 gramsof benzene to form 369 grams of benzene hexachloride, the chlorination was again discontinued, enough benzene makeup added to give 148 grams of unreacted benzene, and chlorination again continued until 111 grams had reacted with 39 grams of benzene to give 150 grams more of benzene hexachloride before stopping again. give 209 grams of unreacted benzene and chlorination continued until 327 grams of chlorine had reacted with 120 7 62.6. The total charge from the chlorination was stirred at about 30 degrees centigrade, and was divided into three samples of approximately 1000 grams each. 500 grams of the adjusted benzene hexachloride slurry were added to a one-liter round bottom flask, brought to 30 degrees centigrade with stirring, and held at this temperature for a few minutes. This slurry was then heated to 80 degrees centigrade in 30 minutes with stirring and held at this temperature for 10 minutes to effect complete solution of the solids. The warm product was then cooled to'plus 9 degrees centigrade in 30 minutes and held at this temperature for Tminutes, until the system had come to Y I equilibrium, as determined by the changes in its specific gravity. This slurry was then'filtered in six and one-half minutes maintained at plus 9 degrees centigrade with ice water.

304 grams of filtrate were obtained and contained 67 grams of solids which analyzed as follows: alpha, 28.8 percent; gamma, 49 percent; delta, 15.2 percent; epsilon, 4.5 percent; and beta, 2.7 percent (by difierence). Based upon solubility data, the mother liquor left in the filter cake was calculated to be 68 grams. The filter cake was washed four times, at which point it weighed 134 grams and analyzed as follows: alpha, 93.0 percent; gamma, 1.6 percent; delta, 1.9 percent; epsilon, 0.8 percent; and beta, 2.7 percent (by difference).

It is to" be understood that when using the word inhibitors we mean those compounds which retard, alter or otherwise prevent the addition reaction between chlorine and benzene.

. .We claim:

In a process for the production of the gamma isomer of benzene hexachloride by the light-catalyzed addition chlorination of benzene, the steps which include: introducing chlorine into a solution of benzene in acetylene tetrachloride containing from about 2 to about 15 percent by weight of unreacted benzene at a rate such that a ratio of gamma isomer to delta isomer produced is maintained between 2.0 to 1.0 and 4.0 to 1.0; and, irradiating the reaction mixture with actinic light having an intensity of about 75 to 9000 foot candles.

References Cited' in the fileof this patent 1 v UNITED STATES PATENTS 2,529,803 Gonze Nov. 14, 1950 2,717,238. Naubauer et al. Sept. 6, 1955 FOREIGN PATENTS 656,457 Great Britain Aug. 22, 1951 678,577 Great Britain Sept. 3, 1952 Finally, a fourth addition was made to,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 ,006 ,833 October 31 1961 Aylmer H. Maude et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 52 and 53, for "vertification" read verification column 2, line 61, for "alph" read alpha column 5, line 21, for "round-botom" read round-bottom line 42, for "alalyzed" read analyzed line 53, after "decreases" insert with increase column 6, line 10, for "alaysis" read analysis column 9, line 27, for "wheels" read wheel line 56 for "is" read in column 10, line 12, after "do" insert not line 24, for "throughly" read thoroughly Signed and sealed this 10th day of August 1965. (SEAL) v Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitcsting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 ,006 ,833 October 31 1961 Aylmer H. Maude et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 52 and 53, for "vertification" read verification column 2, line 61, for "alph" read alpha column 5, line 21, for "round-botom" read round-bottom line 42, for "alalyzed" read analyzed line 53, after "decreases" insert with increase column 6, line 10, for "alaysis" read analysis column 9, line 27, for "wheels" read wheel line 56 for "is" read in column 10, line 12, after "do" insert not line 24, for "throughly" read thoroughly Signed and sealed this 10th day of August 1965. (SEAL) v Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitcsting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,006,833 October 31, 1961 Aylmer H. Maude et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, lines 52 and 53, for "vertification" read verification column 2, line 61, for"alph" read alpha column 5, line 21, for "round-botom" read round-bottom line 42, for "alalyzed" read analyzed line 53, after "decreases" insert with increase column 6, line 10, for "alaysis" read analysis column 9, line 27, for "wheels" read wheel line 56, for "is" read in column 10, line 12, after "do" insert not lim 24, for "throughly" read thoroughly Signed and sealed this 10th day of August 1965. (SEAL) V Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDER Attcsting Officer 

